1. Field of the Invention
The present invention relates to a substrate holder and a substrate holding method for holding a thin plate-like substrate, such as a semiconductor wafer, by vacuum suction.
2. Description of the Related Art
A substrate holder which holds a surface of a substrate by vacuum suction is typically used as a device for holding a thin plate-like substrate, such as a semiconductor wafer, when processing the substrate. A conventional vacuum suction type substrate holder includes a single vacuum production hole, and holds the substrate by producing vacuum in a space between a suction member and the substrate. Further, the conventional substrate holder is configured to eject a high-pressure fluid from the vacuum production hole in order to release the substrate, which has been attracted once, from the substrate holder.
Functions required for the substrate holder include releasing the substrate without putting stress on the substrate, as well as holding the substrate sufficiently. In order to satisfy such requirements, various types of suction members and various shapes of suction surfaces of the substrate holder have been proposed. Especially, as a semiconductor wafer has a thickness of as thin as 1 mm or less and is made of brittle material, careful treatment is necessary so as not to damage the semiconductor wafer when releasing it.
FIG. 1 is a cross-sectional view showing a conventional substrate holder, and FIG. 2 is a top plan view showing the substrate holder in FIG. 1 in a state of not holding the substrate. The substrate holder has a suction stage 200 having a flat and circular top surface and a suction pad 201 attached to the top surface of the suction stage 200. The suction stage 200 is mounted on a rotatable shaft 202, so that the suction stage 200 and the shaft 202 rotate in unison. The shaft 202 has a hollow structure, and a tube 203 extends through the shaft 202. The tube 203 is coupled to the suction stage 200 in an air-tight manner by an O-ring 204. The suction stage 200 is secured to the shaft 202 by four screws 205.
The suction pad 201 is composed of a plurality of pad pieces arranged along a plurality of concentric circles (virtual circles). An outermost pad piece 201a having a closed ring shape is arranged along an outermost periphery of the top surface of the suction stage 200. A single vacuum production hole 206 is formed at a center of the suction stage 200. The vacuum production hole 206 communicates with a recess formed between the pad pieces of the suction pad 201 and also communicates with the tube 203. When the vacuum production hole 206 is evacuated with the substrate W on the suction pad 201, the outermost pad piece 201a of the suction pad 201 functions as a sealing member, whereby negative pressure is created between the substrate W and the suction stage 200 to attract the substrate W.
When releasing the substrate W, fluid communication between the vacuum production hole 206 and a fluid supply source is established via the tube 203. A fluid, such as nitrogen gas, spouts from the vacuum production hole 206 to thereby release the substrate W. However, once one fluid path running from the vacuum production hole 206 to the periphery of the substrate holder is formed during supply of the fluid, most of the fluid supplied is discharged to the exterior through the fluid path. Consequently, it becomes difficult to separate the substrate W from portions of the suction pad 201 other than the fluid path. Especially, in a case of using foam material for the suction pad 201, a portion thereof where the suction pad 201 is in contact with the substrate W microscopically acts as a suction cup, which makes it difficult to release the substrate W from the suction pad 201.
FIG. 3 is a cross-sectional view showing another example of a conventional substrate holder. FIG. 4 is a top plan view of the substrate holder in FIG. 3 in a state of not holding the substrate. As shown in FIG. 4, the top surface of the suction stage 200 has grooves 208 extending radially as illustrated by dashed lines in FIG. 4. These grooves 208 are in fluid communication with the vacuum production hole 206 provided in the center of the suction stage 200. The suction pad 201 is attached to the top surface of the suction stage 200 so as to cover the grooves 208. The suction pad 201 has a plurality of through-holes 209 arranged along the grooves 208, and the grooves 208 and the through-holes 209 communicate with each other.
Even in the case of providing the plurality of through-holes 209 as shown in FIG. 4, once one fluid path is formed between the substrate W and the suction pad 201 when releasing the substrate W, the fluid from all of the through-holes 209 converges on this fluid path. As a result, the same problem arises. Further, once one fluid path is formed, pressure of the fluid supply source is lowered. Consequently, the action of separating the substrate from portions other than the fluid path is weakened.